1. Field of the Invention
This invention relates to a waterless planographic printing plate, i.e. a printing plate suitable for use in planographic printing, wherein need for conventional dampening with an aqueous fountain solution in the printing operation is obviated.
2. Description of the Prior Art
In conventional planographic printing, a printing plate is first dampened with an aqueous fountain solution in order to prevent ink from wetting the nonimaged areas of the printing plate, after which ink is rolled over the plate. The conventional planographic printing has some difficulties inherent in having both ink and a fountain solution. That is, first, the fountation solution applied to the printing plate flows back into the train of inking rollers on the press, causing emulsification of the ink. Secondly, control of the delicate balance between the amounts of ink and a fountain solution applied is difficult to maintain, and thus, the image fidelity and uniformity are difficult to maintain. Thirdly, the fountain solution tends to flow forward over the offset cylinder, moistening the copy paper and causing its dimensional change. Fourthly, in the case where the printing plate is imaged directly by electrophotography, the imaged printing plate must be subjected to an etching treatment and the printing operation becomes complicated.
Many attempts to avoid the above-mentioned difficulties have been made in which a printing plate with background areas that are ink-repellent, without being dampened by an aqueous fountain solution, is used. In general, such a printing plate comprises an overlying surface layer coated on a substrate of the printing plate, which layer is comprised of an ink-repellent material such as an organosilicone polymer or an organofluorine compound. The ink-repellent background areas are formed, for example, by a method wherein an unimaged printing plate having a substrate/photosensitive material/ink-repellent material triple layer structure is imaged and, then, either the imaged areas or the non-imaged areas are removed by a developing solution. Another method for forming the ink-repellent background areas involves depositing an ink-accepting particulate material (commonly referred to in the trade as "toner") onto an unimaged printing plate having a substrate/ink repellent double layer structure in image configuration, followed by heating the toner, thereby to fix it to the printing plate. Tha latter method is conveniently used for in-plant or in-house printing. However, since the ink-accepting toner is not firmly attached to the organosilicone polymer or organofluorine compound overlying layer, the imaged printing plate is poor in printing endurance, i.e., the toner is likely to become removed after a short run on a printing press.
In order to obviate the above-mentioned defect, some proposals have been made to form an overlying modified organopolysiloxane polymeric material layer on a substrate. For example, Japanese Laid-open Patent Application 1803/1975 discloses a planographic printing master having an overlying heterophase polymeric composition layer coated on a substrate, which polymeric composition is predominantly comprised of an organopolysiloxane copolymer, particularly a tri-block copolymer expressed by the formula "ABA" or a multi-block copolymer expressed by the formula (AB)n, wherein one of the A and B is a silicone phase, such as an organopolysiloxane, and the other is a non-silicone phase, such as plystyrene. U.S. Pat. Nos. 4,009,032 and 4,030,416 disclose a planographic printing master having an overlying siloxane block copolymer layer coated on a substrate, which siloxane block copolymer is comprised of siloxane blocks cured to an elastomeric ink releasing condition and organic thermoplastic blocks which are ink accepting. The overlying modified organopolysiloxane polymeric material layers disclosed in these prior art references exhibit enhanced adhesion to toner as compared with an overlying unmodified organopolysiloxane polymeric material layer. However, control of the delicate balance between the background contamination in non-image areas of prints and the printing endurance of the printing plate is difficult to maintain. This is because, in general, the lower the background contamination in non-image areas of prints, the lower the printing endurance of the printing plate. Furthermore, the modified organopolysiloxane polymeric material contains an organopolysiloxane in a relatively large proportion, which is expensive. Cumbersome polymerization procedures are necessary for the preparation of the modified organopolysiloxane polymeric material.
It has also been proposed in Japanese Laid-open Patent Application 29,305/1977 to employ an overlying organopolysiloxane elastomer layer which contains a minor amount of an organosiloxane unit having a reactive organic radical. This proposed method provides a printing plate of improved printing endurance, from which prints of reduced background contamination are obtainable. However, control of the delicate balance between the background contamination in prints and the printing endurance of the printing plate is still difficult to maintain. Furthermore, the organosiloxane having a reactive organic radical is costly in its production.